Electric furnace system



Dec. 5, 1933. A, HANFF 1,937,696

ELECTRIC FURNACE SYSTEI Original Filed April 10. 1930 3 Sheets-Sheet 1 INVENTOR W a. M 7

Dec. 5, 1933. E, A. HANFF 1,937,696

ELECTRIC FURNACE SYSTEM Original Filed April 10, 1930 3 Sheets-Sheet 2 Dec. 5, 1933. v FF 1,937,696

ELECTRIC FURNACE SYSTEM Original Filed April 10, 1930 s sheets-sheet 3 INVENTOR Wav W" JM Patented Dec 5 1933 U ITED STATES PATENT" OFFICE:

ELECTRIC FURNACE SYSTEM Edward A. Hanlf, Pittsburgh, Pa. Original application April 10, 1930, Serial No. 443,089. Divided and this application February 25, 1931. Serial No. 518,121

6 Claims., (Cl. 13-34) My invention relates to apparatus for controlling electric current supplied to a consuming device and, in particular, to the control of current supplied to an arc melting furnace.

This application is a division of my copending application, Serial No. 443,089, filed April 10, 1930, for Electric control systems.

It has previously been the practice to provide each melting furnace with a separate power transformer adapted to reduce the supply line voltage to the operating voltage of the furnace. Some flexibility in the operating voltage is desirable and different voltages have been obtained by changing the connections of the primary winding or by multi-voltage taps. Because of the large currents to be handled, it is not practicable to obtain diiferent voltages by changing the connections of the transformer secondary.

The previous practice has also been character'- ized by the use of high voltage circuit breakers mounted separately from the transformer housing for effecting the desired changes in the connections of the transformer primary. This arrangement necessitates that all the equipment and connections be insulated for the high voltage being handled, and this fact introduces considerable complexity and great expense into the installation.

In operating arc furnaces, it has been found necessary to provide external reactors to limit the current supplied to the furnace on starting. It is desirable, furthermore: to reduce the amount of the external reactance when the voltage on the furnace is reduced. This has been accomplished heretofore by means of taps on the reactor windings or by shunting the external reactance entirely. The present invention includes means for reducing the effectiveness of the reactors without necessitating that taps be provided thereon.

I have invented a control system which is much simpler than those heretofore employed and is, therefore, less expensive to install and requires less space. In addition, the system contemplated by the invention makes provision for the reduction of protective reactance inserted in the power circuit by the same operation which changes the connections of the primary windings. Another characteristic of the invention is that its use is not as hazardous as has been the case, since most of the switching equipment is guarded against accidental contact therewith by an operator.

According to the invention, I provide a power transformer for each arc melting furnace. The external reactance in the form of reactor coils is mounted within the transformer case, as are also the switches employed for changing the connections of the primary winding, toobtain difterentvoltages on the secondary winding. These switches are operable from outside the transformer case and, as a safety measure, I provide that no one of the switches can be operated for varying the magnitude of change so effected.

For a clear understanding of the invention, reference is made to the accompanying drawings, in which Figure-1 is a schematic diagram of one embodiment of the invention;

Figure 2 is a similar illustration of a modified form; t

Figures 3, 4 and 5 are schematic diagrams showing the changes in the connections of the primary windings which are connected by the operation of the system shown in Figure 1, and

Figure 6 is an illustration similar to Figure 1 of a modified form of the invention.

The drawings are intended merely by way of example and not as limitations, since the invention may be practiced in forms other than those shown, without departing from the spirit thereof or the scope of the broader claims.

Referring in detail to the drawings, and for the present to Figure 1, an electric arc furnace is illustrated at 10. The electrodes of the furnace 10 are supplied with current from the secondary windings 11, 12 and 13 of a transformer 14, the secondary windings being connected in delta. The transformer 14 also includes high voltage primary windings 15, 16 and 17. These windings are provided with taps numbered from 1 through 5, and tap changing links 18 and 19 are supplied with each primary winding for varying the number of turns thereof in circuit.

Reactor coils 20, 21 and 22 serve to limit the current supplied to the transformers and the furnace when the latter isstarted.

Current is supplied to the transformer 14 from a supply line 23 through a main circuit breaker 24 which has a closing coil 24c and a trip coil 24t. Tapchanging switches 25, 26 and 27 are provided for altering the connections of the primary windings of the transformer 14, in order to vary the voltage induced in the secondary windings thereof. The switches 25, 26 and 27 have closing and tripping coils indicated by corresponding numerals with the letter 0 for closing coils, and the letter 15 for trip coils. These switches are alsoprovided with ,main contacts indicated by the numeral applied to the particular switch and the number of the switch contact, such as 1, 2, 3 etc. The switch 25, for example, has pivoted con- I tacts 251, 25-2 and 25-3. The switches 25,

26 and 2'7 also operate auxiliary or interlock contacts indicated by the number of the switch with which they are associated, the letter a and. the number of the contact. The switch 25, for example, actuates the interlock contacts 25a1, 25a2 and 23113.

For controlling the energization of the closing and tripping coils ofthe main circuit breakers 24 and the tap changing switches 25, 26 and 27, I provide a series of push button switches which are indicated by the number of the switch with which they are associated, together with the letters pc for the closing switch and pt for the tripping switch.

It will be noted that the reactors, multi-voltage taps and links, and the contadts of the tap-changing switches are mounted within the case of the transformer 14. Only the operating rods of the switches and the low-voltage control circuits are external to the transformer case. This arrangement adds greatly to the simplicity, compactness, low cost and safety of the system.

Having described the construction of the embodiment of the invention shown in Figure 1, the operation thereof is as follows:

The secondary windings 11, 12 and 13 arepermanently connected in delta to the electrodes of the furnace 10. The links 18 and 19 associated with the taps on the windings 15, 16 and 17 are set in the desired position and remain as adjusted without change during the operating cycle of the furnace. It is obvious, of course, that conditions may arise in which it will be found necessary or advisable to change the setting of the tap connecting links 18 and 19. The taps engaged by the links 18 and 19 are selected to give the desired number of turns in the primary windings when the connections including said links are made.

The furnace 10 is started on high voltage by closing the tap changing switch 25 and the main circuit breaker 24. To close the switch 25, the corresponding closing push switch 25100 is operated to bridge its contacts. This operation completes a circuit including the closing coil 250 of.

the switch 25, and an auxiliary source of control current 28 and 29. This circuit may be traced as follows: From the positive terminal 28' of the auxiliary source through the interlock contact 24a1 of the circuit breaker 24 which is closed when the circuit breaker is opened, through the closing coil 250 of the switch 25, the auxiliary contact 26a3 of the switch 26 which is closed when the switch 26 is opened, a similar interlock contact 27113 actuated by the switch 27, the contacts of the push switch 25120, the left hand contacts of the push switches 26pc and 27100, which are normally in the position illustrated, to the negative terminal 29 of the supply source.

It will be seen that this circuit includes interlocks operated by the circuit breaker 24 and the top-changing switches 26 and 27 so that the circuit cannot be completed nor the switch 25 closed if either of the switches 26 and-27 or the circuit breaker 24 is closed. This interlocking increases the safety of operation and eliminates the possibility of damage, due to improper connections. The construction of the switches 25, 26 and 27 and the circuit breaker 24 is such that when they have once been operated to the closed or open position, they remain in that posititon until positively actuated to the other position,

without the use of holding coils or mechanical latches. The circuit including the closing coil and the push switch 2512c need not, therefore, be maintained to keep the switch 25 closed.

When the switch 25 has been closed, the next step in starting the furnace is to close the circuit breaker 24. This is accomplished by closing the push switch 24pc to complete a. circuit including an interlock on one of the switches 25, 26 and 27, andthe closing coil of the circuit breaker 24. This circuit is as follows: From the positive terminal 28 of the auxiliary source through the closing coil 24c of the circuit breaker 24,- through an interlock contact 25a1, which is closed when the switch 25 is closed, through the contacts of the push switch 24pc, and thence to the negative terminal 29 of the control source. Since this circuit includes an interlock operated by the switch 25, it will not be possible to close the circuit breaker 24 until the switch 25 is closed. Upon the completion of this circuit, the circuit breaker 24 is closed and its auxiliary contact 24a1 is opened to prevent the subsequent energization of any of the closing or tripping coils of the tapchanging switches.

When these operations have been completed, the primary windings 15, 16 and 17 of the transformer 14 are connected in delta to the conductors of the supply circuit 23 and the reactors 20, 21 and 22 are connected in series with the phases of the supply circuit. This connection is indicated schematically in Figure 3. As indicated in Figure 1, the phases of the supply circuit 23 are designated by the letters A, B and C. According to Figures 1 and 3, phase conductor A leads. to the reactor 21 which is connected to opposite terminals of the primary windings l5 and 16, the circuit from the reactor 21 to the winding 15 being completed through the contact 25-3 of the switch 25 and the tap changing link 18 which is positioned on tap 2. The reactor 20 is likewise connected to phase conductor C and to opposite terminals of the windings 15 and 17, the connection to the winding 17 being established through the switch contact 25--1, and the link 18. The remaining terminals of the windings 16 and 17 are connected to the reactor 22, the connection from the winding 16 to the reactor 22 being established through the contact 25-2.

The secondary windings 11, 12 and 13 permanently connected in delta to the electrodes of the furnace 10 now supply current to the furnace, at a voltage dependent on the voltage of the supply circuit 23 and the ratio between the number of turns in the primary windings between the left hand terminal and tap 2, and the number of turns in the secondary windings. The reactors 20, 21 and 22 serve to limit the current peak when the furnace is started and, since each reactor carries the current supplied to two transformer windings, the limiting effect thereof is a maximum. The transformer being connected deltadelta, the ratio of phase voltages is simply that of the primary and secondary turns. The line current being 1.73 times the phase current, the effect of the reactors is a maximum.

After the furnace has been started, it will be desirable to operate on lower voltage and to ac complish this it is necessary to change the connections of the primary circuit of the transformer. The first step in effecting this change is the opening of the circuit breaker 24. This may be done by closing the push switch 24m which completes the circuit from the conductor 28 through the tripping coil 24t of the circuit breaker 24, the contacts of the switch 24pt to the conductor 29. The circuit breaker 24 is thus opened and the interlock switch 2401 is again closed. The tap-changing switch 25 is next opened by closing the push switch 25pt. This completes a circuit from the conductor 28 through the interlock 24a1 of the circuit breaker 24, the tripping coil 25t of the tap-changing switch 25, the contacts of the push switch 25pt, the left hand contacts of the push switches 26pc and 2712c to the negative conductor 29. This circuit includes the interlock 24a1 operated by the circuit breaker 24 and the normally closed contacts of the push switches 26:20 and 2'7 90.

"possilzile to open or close any tap-changing while the circuit breaker is closed.

Having re-established no load conditions, the.

This arrangement of the circuit prevents the switch 25 from being opened until the circuitswitch has been opened, and it is likewise imswitch intermediate voltage switch 26 may be closed by operations similar to those disclosed by the switch 25. The push switch 26pc completes a circuit including the interlock contact 24:11, the closing coil 26c ofthe switch 26, the interlock 2'7a2 of the switch 2'7, the interlock contact 25a3 of the switch 25, the right hand contacts of the push switch 2612c and the left hand contacts of the push switch 27:20. When the switch 26 has been closed, the circuit breaker 24 may be reclosed to supply energy to the furnace.

Under these conditions, the circuits established are such as are indicated schematically in Figure 4. The phase conductor A is connected through the contact 26-3 to tap 5 through switch 26-3 and link 19 to tap 5 of the winding 15. The conductor A is also connected directly to the reactor 21. The winding 15 is connected 'to the reactor 20 and the latter is connected through the contact 26--1 and link 19 to the winding 1'7. The phase conductorG is connected ,to the common terminal of the reactor 20 and the winding 1'7. The otherend of the winding 1'7 is connected to the reactor 22 which is connected to phase conductor B, and through the contact 262 and link 19, to the winding 16.

Since the links 19 are adjusted to engage taps 5, the secondary voltage is .less than when the switch 25 was closed. The reactors 20, 21 and 22, furthermore, are connected inside the delta and not in series with the phase conductors, so that with normal load on the transformer, each reactor will carry only 58% of line current. The reactive drop and the percentage of reactance is thus reduced, and since the load on the transformer may be reduced when using the intermediate voltage, the resulting current reduction will further limit the reactive drop. Any one ofthe flve transformer taps may be employed in the intermediate connection, as shown in Figure 4.

If it is desired to reduce the voltage between the furnace electrodes still further, the circuit breaker 24 must be opened and the switch 26 likewise.

These operations are performed in the mannerwhich has already been described. If the switch 27120 is open so that its left hand contacts are closed, and if the circuit breaker 24 is opened, the closing of the switch 26pt will energize the trip coil 26t and open the switch 26.

When the switch 26 and the circuit breaker 24 have been opened, the operation of the push It is thus impossible to close one tape kchanging switch before the previously closed in Figure 5. The phase conductor A is connected to the reactor 21, which in turn is connected to the winding. 16.- This winding is connected through the link 18, a link 30 and the contact 27-4 to a common conductor 31. The phase conductor B is connected to the reactor 22 which is connected to the winding 17. The other end of the winding is connected through its links 18 and 30 and switch contact 2'72 to the conductor 31. Conductor C leads to the reactor 20 connected to the winding 15. The winding 15 is likewise connected through its links 18 and 30 and the switch contact 276 to the conductor 31. The reactors 20, 21 and 22 are rendered ineffective by the switch contacts 2'7-1, 27-5 and 27--3 respectively, which serve to shunt the reactors entirely out of the circuit. The links 30 permit the low voltage connection to be established either through the taps 52, to which the links 18 are adjusted, or the tabs 5 which the links 19 engage.

The switch 27 thus establishes a star delta connection of the transformer through the tap 2 of the primary winding. The secondary voltage is thereby reduced in the ratio of 1 divided by 1.73, and the reactors are shunted so that they are entirely ineffective. The external reactive drop is thus decreased to zero. It is not essential, however, to shunt the reactors, since the current therethrough is reduced in the same proportion as the secondary voltage and the reactor drop will, therefore, be less than in the high voltage connection established when the switch 25 is closed. Summerizing, in the high voltage connection the primary windings are connected in delta with the. reactors in series with the lines. When the intermediate switch is closed, the primary windings are still connected in delta, but to a different tap, preferably one which will give a low secondary voltage. The reactors are shifted from the lines to the inside of the delta in series with the transformer windings individually. In the lower voltage connections established by the switch 2'7, the primary windings are connected in star and the reactors may or may not be shunted.

Referring to Figure 2, I have illustrated a somewhat simpler control system making use of manual switches and mechanical interlocks. Corresponding elements are indicated by the same ref erence numerals indicated in Figure 1. A supply of energy from the circuit 23 'to the furnace 10 is controlled by a manually operable circuit breaker 32 and tap-changing switches 33 and 34 which provide high and low voltages, respectively, between the furnace electrodes. In order to interlock the circuit breaker 32 and the switches 33 and 34, I provide a bell crank 35 and 'a sliding bar 36 arranged to be actuated by movement of the circuit breaker 32. The bar 36 has openings 3'7 and 38 therein for receiving discs 39 and 40 secured to the operating shafts of the switches 33 and 34. The rod 36 slides in bearings 41 and 42. A walking beam 43 is pivoted at its center and has forked ends encircling the ends of the switches 33 and 34. The bar 36 and the discs 39 and 40 cooperate to prevent movement: of the switches 33 and 34 in either direction when the circuit breaker 32 is closed. The walking beam 43 prevents the simultaneous closing 01' both the switches 33 and 34.

When the switch 33 is closed, the windings 15, 16 and 17 are connected in delta with the reactors 20, 21 and 22 in series with the phase conductors A, B and C leading to the corners of the delta, as shown in Figure 3. The links 18 engage the taps which include the minimum number of primary turns in circuit, so that a high voltage is induced in the secondary windings 11, 12 and 13.

When the switch 34 is closed, the primary windings are again connected in delta but with the reactors inside the legs thereof, as illustrated in Figure 4. In this case the links 19 and the taps 3 are included in the circuits and a lower voltage is induced in the secondary windings.

The system shown in Figure 2 thus provides manually operated equipment for controlling the voltage in two steps.

Figure 6 illustrates diagrammatically a system which is similar to that shown in Figure 1, except that rotary tap changing switches 44, 45 and 46, driven by a pilot motor 47, are employed instead of the individually operable solenoid type of switches shown in Figure 1. Reactors 48, 49

'and 50 having variable taps are utilized in connection with the system of Figure 6. The operation of the motor 4'7 is controlled by a series of switches 48', 49' and 50 cooperating with a limit switch 51 driven by the motor. Push switches 24100 and 24pt control the circuit breaker 24 as in Figure 1.

The rotary tap changing switches 44, 45 and 46 each consist of a contact segment 52 and contact points 53, 54, 55, 56, 57 and 58. The tap changing switches and the limit switch are mounted on a common shaft driven by the motor 47 through a worm and worm wheel 59. The switches 44, 45 and 46 serve the same purpose as the individual switches shown in Figure 1, namely, to alter the connections in the primary circuit to give various secondary voltages.

The limit switch 51 comprises a rotating arm 60 which makes sliding engagement with contact segments 61, 62 and 63, by means of contact buttons 64 and 65 which are electrically connected. A bridging contact 66 on the arm 60 is adapted to engage contact buttons 67, 68 and 69 positioned between the ends of the segments 61, 62 and 63.

When the tap-changing switches 44, 45 and 46 have the position illustrated, the circuit breaker 24 may be closed by operating the push switch 24110. As shown in Figure 6, the bridging contact 66 engages the buttons 69 so that the operation of the push switch 24pc completes a circuit from the conductor 28 through the closing coil 240, buttons 69 and the bridging contact 66, the contact of the switch 24320 to the conductor 29. The circuit breaker is thereby closed. In the illustrated position of the tap-changing switches, connections are made so that the primary windings 15, 16 and 17 are connected in delta and the reactors 48, 49 and 50 are in series with one of the primary windings. The reactor 48 is connected to the contact point 53 of the switch 46 through a link 70 and the segment 52 of the switch 46 completes the connection to the link 18 and tap 2 of the winding 17. The winding 17 is also connected to the phase conductor C to which the reactor 50 is also connected. The reactor 50, through the link '70 and switch 45 is connected to the winding 16 and the reactor 49 is likewise connected to the winding 15 through the switch 44. This arrangement of the switches gives the maximum secondary voltage.

When it is desired to reduce the voltage between electrodes in the furnace, the circuit breaker 24 must first be opened. This is accomplished by closing the push switch 24pt which completes a circuit through the tripping coil 24t of the circuit breaker 24. The auxiliary contacts 24a2 and 24113 are simultaneously closed. The push switch 49 is now operated to complete a circuit from the conductor 28 through the contact 24a3 to a solenoid operated motor switch 71. The circuit extends to the buttons 64 and 65 of the arm 60 and thence to the segment 62, to the switch 49 and through the left hand contacts of the switches 48 and 24190 to the negative conductor 29. The closing of the switch 49 also sets up a circuit including the segment 61 so that, if the bridging contact 66 were in engagement with the contacts 68, the switch 71 would nevertheless be energized.

The operation of the switch 71 completes a circuit from the conductor 28 through the auxiliary contact 24a2, the upper contact of the motor switch, the field and armature of the motor 47 in series, the lower contact of the motor switch to conductor 29. The motor 47 then rotates to shift the rotary switches 44, 45 and 46 and the limit switch 51. As the switch 51 rotates, the button 65 slides along the segment 62, but before the engagement is broken, the button 64 engages the segment 61 so that the circuit for the motor switch 71 is not interrupted until the bridging contact 66 engages the contacts 67 and the button 64 lies between the segments 61 and 62 which were connected to the power source by the closing of the switch 49'. When the arm 60 reaches 4 such position, the circuit for the motor switch 71 is broken and the switch opens to stop the motor.

The segments 52 of the switches 44, 45 and 46 are thus shifted into engagement with the contacts 54 and 55 of said switches. It is necessary that the push switch 49' be held closed while the movement of the rotary switches takes place, in order to maintain the motor switch 71 energized. It would be a simple matter, however, to provide relays for maintaining voltage on the segments 61 and 62 until the desired movement had taken place without necessitating more than a momentary closure of the switch 49'.

When the circuit breaker 24 is closed, energy is again supplied to the furnace 10 but the secondary voltage of the transformer 14 is lowered. The taps 3 are connected through links 19 to the contacts 55 of the rotary switches and links 72 connect a portion of the reactors 48, 49 and 50 in L series with the transformer primaries between the phase conductors A, B and C. The circuit for the winding 15, for example, extends from the conductor A through the winding, tap 3, link 19, contact 55, segment 52 and contact 54 of switch 44,

circuit. The effect of the reactors has been reduced by cutting out a portion of each reactor.

When it is desired to reduce the secondary voltage further, the switch 50 is operated and, in the manner previously described, causes the segments 62 and 60 to be energized so that the bridging contact 66 is operated to a position therebetween. The segments-62 of the switches 45, 46 and 47 are thus shifted to engage contacts 56 and 57. In this position of the tap changing switches, the windings of the transformer may be connected through either the links 18 or the links 19 by means of links '73 which connect the 1 contact 56 to either of the contacts 55 and 58.

In either case, the windings are connected direct- 1y to a common conductor '74 through the contacts 56 and 57 of the segment 62. The reactors 48, 49 and 50 are left out of the circuit and the secondary voltage is decreased further by the star delta connection which is effective the lower position of the segments 52. I

The system shown in Figure 6 is entirely automatic and any changes desired may be effected merely by pressing appropriate push switches.

. This causes the rotary tap-changing switches to move into the desired position where they are automatically stopped and prepared for any subsequent movements. The closing of the push switch 48, for example, will restore the conditions illustrated by energizing the segments 61 and 63. If the switch 48 is closed, however, when tap changing switches occupy the illustrated positions, nothing will happen since the circuit for the motor switch '71 will not be completed, the segments 62 being de-energized and the contact button 64 being out of engagement with the contact segment 61 and 63.

By interlocking the push switches 48', 49' and 50' and the circuit breaker control switch 24pc, the operation of the tap-changing switches, while the circuit breaker is closed, is prevented as is also the initiation of any change-in the position of the switches 44, 45 and '46 before all push switches are restored to normal. The control switches for the tap-changing switches in Figure 1, as well as those in Figure -6, may be spring restored or they may occupy either open or closed positions until positively actuated therefrom.

It will be apparent that the invention provides a simple, compact and safe means for controlling the voltage between electrodes of an electric furnace. The system, furthermore, is very flexible and may be adjusted to meet a large variety of operating requirements. The secondary voltages developed when the various schemes of connection are employed may be altered by means of the taps 18, 19, 30,- 70, '72 and '73, so that a large number of variations may be obtained.

The interlocking of the various operations makes it practically impossible to cause an operation which would introduce any hazard to the equipment or the operators, and the unitary mounting of the transformer windings, reactors and tap-changing switches. reduces the expense of installing the system, the amount of floor space required and the possibility of accidents which is always present in unprotected circuits and apparatus. By means of the numerous connection links, any desired transformer taps or reactor taps may be connected at each position of the tap-changing switch. The system may thus be modified to suit conditions existing in any pull switches, etc. The external switching means indicated in Figures 1, 2 and 6, furthermore, can be adapted to hand operation, solenoid operation or motor driven rotary operation. 1

Although I have illustrated and described but a few preferred. embodiments of the invention, I do not intend to be limited to the-specific details thereof, since the invention'may be otherwise embodied thescope'of thefollowing claims, without sacrificing the advantages. resulting therefrom.

I claim: 1. A method of controlling the current supplied to electric furnace electrodes from the secondary windings of a transformer, which includes the steps of connecting the transformer primaries in delta and connecting reactance coils in series with the. supply conductors leading to the delta connected primary windings for a relatively high secondary voltage, and connecting said reactance coils inside the delta connections for a lower secondary voltage.

2. In a method of controlling the current supplied to an electric furnace from a three-phase transformer, the steps including connecting the transformer primary windings in delta, connecting the delta to a three-phase source in series with reactors, for a high secondary voltage, connecting the reactors inside the delta for a lower secondary voltage, and connecting the primary windings in star for a still lower secondary voltage.

3. In a method of controlling the current supplied to an electric furnace from a three-phase transformer, the steps including connecting the transformer primary windings in delta, connecting the delta to a three-phase source series with reactors, for a high secondary voltage, connecting the reactors inside the delta and reducing the eflect of said reactors for a lower secondary voltage, and connecting the primary windings in star for a still lower secondary voltage.

4. In a method of controlling the current supplied to-an electric furnace from a threephase transformer, the steps including connect- 120 ing the transformer primary windings in delta, connecting the delta to a three phase source in series with reactors, for a high secondary voltage, connecting the reactors inside the delta for a lower secondary voltage, and connecting the 125 primary windings in star and shunting said reactors for a still lower secondary voltage.

5. In a method of controlling the current supplied to an electric furnace from a three-phase transformer, the steps including connecting a 130 portion of each of the transformer primary windings in a delta and connecting the delta to a three-phase source in series with reactors for a the delta connections.

EDWARD A. HANFF. 

